Semiconductor package and method for fabricating base for semiconductor package

ABSTRACT

The invention provides a semiconductor package and a method for fabricating a base for a semiconductor package. The semiconductor package includes a conductive trace embedded in a base. A semiconductor device is mounted on the conductive trace via a conductive structure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of pending U.S. application Ser. No. 13/721,983, filed on Dec. 20, 2012, which claims the benefit of U.S. Provisional Application No. 61/677,835, filed on Jul. 31, 2012, the entireties of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a semiconductor package and a method for fabricating a base for a semiconductor package, and in particular, to a base for a high density semiconductor package.

Description of the Related Art

In order to ensure miniaturization and multi-functionality of electronic products or communication devices, semiconductor packages are desired to be small in size, to support multi-pin connection, to support high speeds, and to support high functionality. The demand for increasing Input-Output (I/O) pin counts and high-performance ICs has led to the development of flip chip packages.

Flip-chip technology uses bumps on a chip to interconnect to a package substrate. The flip-chip is bonded face down to the package substrate through the shortest path. The technology used can be applied not only to a single-chip package, but also to higher or integrated levels of packaging in which the packages are larger and packaged with more sophisticated substrates that accommodate several chips to form larger functional units. The flip-chip technique, using an area array, can achieve a high density interconnection with devices and a very low inductance interconnection with packaging. However, this requires printed circuit board (PCB) fabricators to minimize line widths and space or to develop direct chip attach (DCA) semiconductors. Accordingly, the increased amount of input/output connections of a multi-functional flip-chip package may induce thermal electrical problems, for example, problems with heat dissipation, cross talk, signal propagation delay, electromagnetic interference for RF circuits, etc. The thermal electrical problems may affect the reliability and quality of products.

Thus, a novel high-density flip chip package and a printed circuit board for a high-density flip chip package are desirable.

BRIEF SUMMARY OF INVENTION

A semiconductor package and a method for fabricating a base for a semiconductor package are provided. An exemplary embodiment of a semiconductor package includes a conductive trace embedded in a base. A semiconductor device is mounted on the conductive trace via a conductive structure.

Another exemplary embodiment of a semiconductor package includes a conductive trace, having a bottom surface and at least a portion of a sidewall connected to a base. A semiconductor device is mounted on the conductive trace via a conductive structure.

An exemplary embodiment of a method for fabricating a base for a semiconductor package includes providing a carrier with conductive seed layers on a top surface and a bottom surface of the carrier. First conductive traces are respectively formed on the top surface and the bottom surface of the carrier, connecting to the conductive seed layers. A first base material layer and a second base material layer are respectively laminated on the top surface and the bottom surface of the carrier, covering the first conductive traces. Second conductive traces are respectively formed on first surfaces of the first base material layer and the second base material layer, wherein the first surfaces of the first base material layer and the second base material layer are respectively away from the top surface and the bottom surface of the carrier. The first base material layer with the first and second conductive traces thereon and the second base material layer with the first and second conductive traces thereon are separated from the carrier to form a first base and a second base.

Another exemplary embodiment of a method for fabricating a semiconductor package includes providing a base, forming a conductive trace on the base, further forming an additional insulation material on the base, and further defining patterns upon the additional insulation material, wherein the pattern is formed on at least one conductive trace.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIGS. 1 to 4 show cross sections of various exemplary embodiments of a semiconductor package of the invention.

FIGS. 5a to 5e are cross sections showing one exemplary embodiment of a method for fabricating a base for a semiconductor package of the invention.

FIGS. 6a to 6e are cross sections showing another exemplary embodiment of a method for fabricating a semiconductor package of the invention.

DETAILED DESCRIPTION OF INVENTION

The following description is a mode for carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. Wherever possible, the same reference numbers are used in the drawings and the descriptions to refer the same or like parts.

The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto and is only limited by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual dimensions to practice of the invention.

FIGS. 1 to 4 show cross sections of various exemplary embodiments of a semiconductor package of the invention. In this embodiment, the semiconductor package can be a flip chip package using conductive structures, for example, copper pillar bumps, connecting a semiconductor device to a base. Alternatively, the semiconductor package can be a package using wire bonding technology to connect a semiconductor device to a base. FIG. 1 shows a partial cross section of one exemplary embodiment of a semiconductor package 500 a of the invention. Please refer to FIG. 1, wherein the semiconductor package 500 a comprises a base 200 having a device attach surface 214. In one embodiment, the base 200, for example, a print circuit board (PCB), may be formed of polypropylene (PP). It should be also noted that the base 200 can be a single layer or a multilayer structure. A plurality of conductive traces 202 a is embedded in the base 200. In one embodiment, the conductive traces 202 a may comprise signal trace segments or ground trace segments, which are used for input/output (I/O) connections of a semiconductor device 300 mounted directly onto the base 200. Therefore, each of the conductive traces 202 a has a portion serving as a pad region of the base 200. In this embodiment, the conductive traces 202 a are designed to have a width which is larger than 5 μm. However, it should be noted that there is no limitation on the width of the conductive traces. For different designs, the width of the conductive traces can be smaller than 5 μm if required.

A semiconductor device 300 is mounted on the device attach surface 214 of the base 200 with an active surface of the semiconductor device 300 facing the base 200 by a bonding process. In one embodiment, the semiconductor device 300 may comprise a die, a passive component, a package or a wafer level package. In this embodiment, the semiconductor device 300 is a flip chip package. A circuitry of the semiconductor device 300 is disposed on the active surface, and metal pads 304 are disposed on a top of the circuitry. The circuitry of the semiconductor device 300 is interconnected to the circuitry of the base 200 via a plurality of conductive structures 222 disposed on the active surface of the semiconductor device 300. However, it should be noted that the conductive structures 222 shown in FIG. 1 is only an example and is not a limitation to the present invention.

As shown in FIG. 1, the semiconductor device 300 may include a body 301, metal pads 304 overlying the semiconductor body 301, and an insulation layer 302 covering the metal pads 304. In this embodiment, the semiconductor body 301 may include but is not limited to a semiconductor substrate, circuit elements fabricated on the main surface of the semiconductor substrate, inter-layer dielectric (ILD) layers and an interconnection structure. In one embodiment, the interconnection structure may comprise a plurality of metal layers, a plurality of dielectric layers alternatively laminated with the metal layers and a plurality of vias formed through the dielectric layers on the semiconductor substrate. The metal pads 304 comprise the topmost metal layer of the metal layers of the interconnection structure. In one embodiment, the insulation layer 302 may be a single layer structure or a multilayer structure, and the insulation layer 302 may comprise but is not limited to silicon nitride, silicon oxide, silicon oxynitride, polyimide or any combination thereof. Also, the insulation layer 302 may have functions of stress buffering and insulation. In one embodiment, the metal pad 304 may comprise but is not limited to aluminum, copper or alloys thereof. A plurality of openings can be formed in the insulation layer 302. Each of the openings exposes at least a portion of one of the metal pads 304.

As shown in FIG. 1, the conductive structure 222 may comprise a conductive bump structure such as a copper bump or a solder bump structure, a conductive wire structure, or a conductive paste structure. In this embodiment, the conductive structure 222 may be a copper bump structure composed of a metal stack comprising a UBM (under bump metallurgy) layer 306, a copper layer 216 such as a plated copper layer, a conductive buffer layer 218, and a solder cap 220. In one embodiment, the UBM layer 306 can be formed on the exposed metal pads 304 within the openings by a deposition method such as a sputtering or plating method and a subsequent anisotropic etching process. The anisotropic etching process is performed after forming conductive pillars. The UBM layer 306 may also extend onto a top surface of the insulation layer 302. In this embodiment, the UBM layer 306 may comprise titanium, copper or a combination thereof. A copper layer 216 such as an electroplated copper layer can be formed on the UBM layer 306. The opening can be filled with the copper layer 216 and the UBM layer 306, and the copper layer 216 and the UBM layer 306 within the opening may form an integral plug of the conductive structure 222. A formation position of the copper layer 216 is defined by a dry film photoresist or liquid photoresist patterns (not shown).

A solder cap 220 can be formed on the copper layer 216 by electroplating a solder with a patterned photoresist layer or by a screen printing process and a subsequent solder re-flow process. A conductive buffer layer 218 formed of Ni may be formed between the copper layer 216 and the solder cap 220 by an electroplating method. The conductive buffer layer 218 may serve as a seed layer, adhesion layer and barrier layer for the solder cap 220 formed thereon. In this embodiment, the conductive structure 222, such as a conductive pillar structure, is used as a solder joint for the metal pad 304, which transmits input/output (I/O), ground or power signals of the semiconductor device 300 formed thereon. Therefore, the copper layer 216 of the conductive structure 222 may help to increase the mechanical strength of the bump structure. In one embodiment, an underfill material or the underfill 230 can be introduced into the gap between the semiconductor device 300 and the base 200. In one embodiment, the underfill 230 may comprises a capillary underfill (CUF), molded underfill (MUF) or a combination thereof.

In one embodiment, the conductive traces may have a top surface disposed above, below or aligned to a surface of the base to improve routing ability for high-density semiconductor packages. As shown in FIG. 1, the conductive traces 202 a have top surfaces 212 a disposed below a device attach surface 214 of the base 200. That is to say, a bottom surface 206 a and at least a portion of a sidewall 204 a of the conductive trace 202 a are designed to be connected to the base 200. In this embodiment, the solder cap 220 of the conductive structure 222 is disposed to contact with a portion of the base 200 and to connect to a top surface 212 a of the conductive trace 202 a only. Due to the top surfaces of the conductive traces being recessed from the device attach surface 214 of the base 200, the bump-to-trace space is increased and the problem of bump-to-trace bridging can be effectively avoided.

FIG. 2 shows a partial cross section of another exemplary embodiment of a semiconductor package 500 b of the invention. Elements of the embodiments that are the same or similar as those previously described with reference to FIG. 1, are hereinafter not repeated for brevity. In this embodiment, conductive traces 202 b of the semiconductor package 500 b embedded in the base 200 may have a top surface 212 b designed to be aligned to a device attach surface 214 of the base 200 to improve routing ability for high-density semiconductor packages. That is to say, a bottom surface 206 b and a sidewall 204 b of the conductive trace 202 b are designed to be fully connected to the base 200. Therefore, the solder cap 220 of the conductive structure 222 is disposed on the device attach surface 214 of the base 200, contacting the top surface 212 b of the conductive trace 202 b only.

FIG. 3 shows a partial cross section of yet another exemplary embodiment of a semiconductor package 500 c of the invention. Elements of the embodiments that are the same or similar as those previously described with reference to FIGS. 1 and 2, are hereinafter not repeated for brevity. In this embodiment, conductive traces 202 c of the semiconductor package 500 c embedded in the base 200 may have a top surface 212 c designed above a device attach surface 214 of the base 200 to improve routing ability for high-density semiconductor packages. That is to say, a bottom surface 206 c and only a portion of a sidewall 204 c of the conductive trace 202 c are designed to be connected to the base 200. Therefore, the solder cap 220 of the conductive structure 222 is disposed on the device attach surface 214 of the base 200, wrapping a top surface 212 c and only a portion of the sidewall 204 c of the conductive trace 202 c.

FIG. 4 shows a partial cross section of still another exemplary embodiment of a semiconductor package 500 d of the invention. Elements of the embodiments that are the same or similar as those previously described with reference to FIGS. 1-3, are hereinafter not repeated for brevity. In one embodiment, the base may comprise a single layer structure as shown in FIGS. 1-3. Alternatively, the base may comprise a multilayer structure. In this embodiment, conductive traces 202 d of the semiconductor package 500 d embedded in the base portion 200 a may have a top surface 212 d designed to be aligned to a surface 214 of the base portion 200 a to improve routing ability for high-density semiconductor packages. That is to say, a bottom surface 206 d and a sidewall 204 d of the conductive trace 202 d are designed to be connected to the base portion 200 a. Also, an insulation layer 208 having openings 210 is disposed on the base portion 200 a. The insulation layer 208 is disposed above the device attach surface 214 of the conductive trace 202 d. In this embodiment, the base portion 200 a and the insulation layer 208 collectively serve as a multilayer base. As shown in FIG. 4, the conductive traces 202 d are exposed within the openings 210. Therefore, the solder cap 220 of the conductive structure 222 is formed through a portion of the insulation layer 208, contacting a top surface 212 d of the conductive trace 202 d only. It should be noted that it is not necessary for the insulation layer 208 to align with the sidewall 204 d of the conductive traces 202 d. Instead, it can be designed to be distanced outward or inward from the sidewall 204 d of the conductive traces 202 d as shown in FIG. 4.

FIGS. 5a to 5d are cross sections showing one exemplary embodiment of a method for fabricating two bases 200 c and 200 d for a semiconductor package of the invention. In this embodiment, the method for fabricating bases for a semiconductor package is also called a double-sided base fabricating process. Elements of the embodiments that are the same or similar as those previously described with reference to FIGS. 1-4, are hereinafter not repeated for brevity. As shown in FIG. 5a , a carrier 400 with conductive seed layers 402 a and 402 b on a top surface 401 and a bottom surface 403 is provided. In one embodiment, the carrier 400 may comprise FR4 glass epoxy or stainless steel. Also, the conductive seed layers 402 a and 402 b are used as seed layers for subsequently formed interconnection conductive traces of bases on the top surface 401 and the bottom surface 403 of the carrier 400. In one embodiment, the conductive seed layers 402 a and 402 b may comprise copper.

Next, as shown in FIG. 5b , first conductive traces 404 a and 404 b are respectively formed on the top surface 401 and the bottom surface 403 of the carrier 400. Bottom portions of the first conductive traces 404 a and 404 b connect to top portions of the conductive seed layers 402 a and 402 b. In one embodiment, the first conductive traces 404 a and 404 b may be formed by a plating process and an anisotropic etching process. The plating process and the anisotropic etching process are simultaneously performed on the top surface 401 and the bottom surface 403 of the carrier 400. In one embodiment, the plating process may comprise an electrical plating process. In one embodiment, the first conductive traces 404 a and 404 b may comprise copper. In one embodiment, the first conductive traces 404 a and 404 b are designed to have a width which is larger than 5 μm. However, it should be noted that there is no limitation on the width of the conductive traces. For different designs, the width of the conductive traces can be smaller than 5 μm if required. In this embodiment, the anisotropic etching process may precisely control the width of the first conductive traces 404 a and 404 b.

Next, as shown in FIG. 5c , a laminating process is performed to respectively dispose a first base material layer 406 a and a second base material layer 406 b on the top surface 401 and the bottom surface 403 of the carrier 400, wherein the first base material layer 406 a and a second base material layer 406 b respectively cover the first conductive traces 404 a and 404 b. In this embodiment, the laminating process of the first base material layer 406 a and the second base material layer 406 b is simultaneously performed on the on the top surface 401 and the bottom surface 403 of the carrier 400. In one embodiment, the first base material layer 406 a and the second base material layer 406 b may comprise polypropylene (PP).

Next, please refer to FIG. 5c again, wherein a drilling process is performed to form openings (not shown) through the first base material layer 406 a and the second base material layer 406 b to define the formation positions of subsequently formed vias 408 a and 408 b. In one embodiment, the drilling process may comprise a laser drilling process, an etching drilling process or a mechanical drilling process. Next, a plating process is performed to fill a conductive material into the openings to form vias 408 a and 408 b for interconnecting the first conductive traces 404 a and 404 b to subsequent second conductive traces 410 a and 410 b. In this embodiment, the drilling process and the plating process are simultaneously performed on the first base material layer 406 a and the second base material layer 406 b, respectively.

Next, please refer to FIG. 5c again, wherein a plurality of second conductive traces 410 a and 410 b are respectively formed on a first surface 412 of the first base material layer 406 a and a first surface 414 of the second base material layer 406 b. As shown in FIG. 5c , the first surface 412 of the first base material layer 406 a and the first surface 414 of the second base material layer 406 b are respectively away from the top surface 401 and the bottom surface 403 of the carrier 400. The second conductive traces 410 a and 410 b are formed by a plating process and an anisotropic etching process. The plating process and the anisotropic etching process are simultaneously performed on the first surface 412 of the first base material layer 406 a and the first surface 414 of the second base material layer 406 b. In one embodiment, the plating process may comprise an electrical plating process. In one embodiment, the second conductive traces 410 a and 410 b may comprise copper. In one embodiment, the second conductive traces 410 a and 410 b are designed to have a width which is larger than 5 nm. However, it should be noted that there is no limitation on the width of the conductive traces. For different designs, the width of the conductive traces can be smaller than 5 nm if required. In this embodiment, the anisotropic etching process may precisely control the width of the second conductive traces 410 a and 410 b.

Next, as shown in FIG. 5d , the first base material layer 406 a with the first and second conductive traces 404 a and 410 a thereon and the second base material layer 406 b with the first and second conductive traces 404 b and 410 b thereon are respectively separated from the top surface 401 and the bottom surface 403 of the carrier 400 to form a first base 200 c and a second base 200 d which are separated from each other. Next, as shown in FIG. 5d again, the conductive seed layers 402 a and 402 b are removed from the first base 200 c and the second base 200 d, respectively.

As shown in FIGS. 5d and 5e , the first conductive traces 404 a and 404 b are aligned to second surfaces 416 and 418 of the of the first and second bases 200 c and 200 d, which are respectively opposite to the first surfaces 412 and 414. In this embodiment, the first base 200 c and the second base 200 d are simultaneously fabricated on opposite surfaces (the top surface 401 and the bottom surface 403) by the double-sided base fabricating process.

Alternatively, two passivation or insulation layers (not shown) having openings may be optionally formed respectively on a second surface 416 of the first base 200 c and the second surface 418 of the second base 200 d after the separation of the first base 200 c and the second base 200 d as shown in FIGS. 5d and 5e . In this embodiment, the first conductive traces 404 a and 404 b of the first and second bases 200 c and 200 d are exposed within the opening. Positions of the insulation layer with openings and the first conductive traces 404 a/404 b as shown in FIG. 5d /5 e can be similar to the insulation layer 208 with openings 210 and the conductive traces 202 d as shown in FIG. 4. Also, in this embodiment, the first base 200 a/second base 200 b and the insulation layer thereon collectively serve as a multilayer base.

FIGS. 6a to 6e are cross sections showing another exemplary embodiment of a method for making a semiconductor package of the invention. Also, FIG. 6e shows a cross section of another exemplary embodiment of a semiconductor package 500 e of the invention. Elements of the embodiments that are the same or similar as those previously described with reference to FIGS. 1-4 and 5 a-5 e, are hereinafter not repeated for brevity. Alternatively, the base may have a multilayer structure. As shown in FIG. 6a , a base 450 with a top surface 451 is provided. Next, as shown in FIG. 6b , at least one conductive trace 454 is formed on the top surface 451 of the base 450. In one embodiment, the conductive trace 454 may be formed by a plating process and an anisotropic etching process. In one embodiment, the plating process may comprise an electrical plating process. In one embodiment, the conductive trace 454 may comprise copper. In one embodiment, the conductive trace 454 is designed to have a width which is larger than 5 μm. However, it should be noted that there is no limitation on the width of the conductive traces. For different designs, the width of the conductive traces can be smaller than 5 μm if required. In this embodiment, the anisotropic etching process may precisely control the width of the conductive trace 454.

Next, as shown in FIG. 6c , a laminating process is performed to respectively dispose an additional insulation material 456 on the top surface 451 of the base 450. Also, the additional insulation material 456 covers a top surface 460 and sidewalls 462 of the conductive trace 454.

Next, please refer to FIG. 6d , wherein a drilling process is performed to form at least one opening 458 through the additional insulation material 456 to define formation of a position of a subsequently formed conductive structure, for example, a copper bump structure or a solder bump structure. In one embodiment, the drilling process may comprise a laser drilling process, an etching drilling process or a mechanical drilling process. In this embodiment, the top surface 460 of the conductive trace 454 is exposed within the opening 458 of the additional insulation material 456.

Next, as shown in FIG. 6e , a bonding process is performed to mount a semiconductor device 300 on the base 450 through the conductive structure 222. Elements of the semiconductor device 300 and the conductive structure 222 that are the same or similar as those previously described with reference to FIGS. 1-4, are hereinafter not repeated for brevity. After the bonding process, the conductive structures 222 are disposed through the opening 458 of the additional insulation material 456, contacting to the top surface 460 of the conductive trace 454 only. Next, an underfill material or the underfill 230 can be introduced into the gap between the semiconductor device 300 and the additional insulation material 456. In one embodiment, the underfill 230 may comprises a capillary underfill (CUF), molded underfill (MUF) or a combination thereof. Finally, the base 450, the additional insulation material 456, the semiconductor device 300, the conductive trace 454, and the conductive structure 222 collectively form a semiconductor package 500 e.

Exemplary embodiments provide a semiconductor package. The semiconductor package is designed to comprise conductive trace embedded in a base, for example, a print circuit board (PCB). The conductive traces may have a top surface disposed above, below or aligned to a surface of the base to improve routing ability for high-density semiconductor packages. Also, the conductive traces are designed to have a width which is larger than 5 μm. Further, the base may comprise a single layer structure or a multilayer structure. Exemplary embodiments also provide a method for fabricating a base for a semiconductor package. In one embodiment, the method can fabricate two bases on two sides of a carrier simultaneously. Also, the conductive traces may be embedded in the base. Further, the conductive trace may be formed by a plating process and an anisotropic etching process, and the anisotropic etching process may precisely control the width of the conductive trace. Alternatively, the method can fabricate a base comprising a single layer structure or a multilayer structure to improve design capability.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1-17. (canceled)
 18. A semiconductor package, comprising: a base comprising a top surface and a recess having an insulative bottom surface below the top surface; a conductive trace embedded in the recess such that the conductive trace is in contact with the insulative bottom surface, wherein a top surface of the conductive trace is below the top surface of the base; and a semiconductor device mounted on the conductive trace via a conductive structure, wherein at least a portion of the conductive structure at or above the top surface of the base has a width that is greater than a width of the conductive trace, wherein the conductive trace is elongated and carries a signal or a ground across at least a portion of the base.
 19. The semiconductor package of claim 18, wherein the conductive structure is in contact with an entire width of the top surface of the conductive trace.
 20. The semiconductor package of claim 18, and wherein the top surface of the conductive trace and a bottom surface of the conductive trace have a same width.
 21. The semiconductor package of claim 20, wherein the width of the top surface of the conductive trace and the bottom surface of the conductive trace is the same as a width of a bottom surface of the recess.
 22. The semiconductor package of claim 18, wherein the insulative bottom surface of the recess and at least one side wall of the recess are formed of a same insulative material.
 23. The semiconductor package of claim 18, wherein the conductive trace is a single layer.
 24. The semiconductor package of claim 23, wherein the base covers an entirety of a bottom surface of the conductive trace underneath the conductive structure.
 25. The semiconductor package of claim 18, wherein the semiconductor device comprises a body having a first surface, a metal pad on the first surface, and an insulation layer covering at least a portion of the first surface of the body and a portion of the metal pad, wherein an exposed portion of the metal pad is mounted on the conductive trace via the conductive structure.
 26. The semiconductor package of claim 18, wherein the conductive structure comprises a conductive pillar structure comprising an under bump metallurgy (UBM) layer, a copper layer, a solder cap and a conductive buffer layer between the copper layer and the solder cap.
 27. The semiconductor package of claim 26, wherein the copper layer has a width that is greater than a width of the conductive trace.
 28. The semiconductor package of claim 26, wherein the UBM layer contacts an exposed portion of a metal pad of the semiconductor device, and wherein the solder cap is in contact with at least a portion of the top surface of the base, at least one side wall of the recess and the top surface of the conductive trace.
 29. The semiconductor package of claim 18, further comprising: an underfill between the semiconductor device and the base.
 30. The semiconductor package of claim 29, wherein the underfill comprises capillary underfill (CUF), molded underfill (MUF), nonconductive paste (NCP), nonconductive film (NCF) or a combination thereof.
 31. A semiconductor package, comprising: a base comprising a recess; a copper trace embedded in the recess such that a top surface of the copper trace is below a top surface of the base, wherein the top surface of the copper trace is exposed within the recess; and a semiconductor device mounted on the copper trace via a conductive structure that is in direct contact with the top surface of the copper trace, wherein at least a portion of the conductive structure at or above the top surface of the base has a width that is greater than a width of the copper trace.
 32. The semiconductor package of claim 31, wherein a bottom surface of the recess and at least one side wall of the recess are formed of a same insulative material.
 33. The semiconductor package of claim 31, wherein the base covers an entirety of a bottom surface of the copper trace underneath the conductive structure.
 34. The semiconductor package of claim 31, wherein the copper trace is elongated and carries a signal or a ground across at least a portion of the base.
 35. The semiconductor package of claim 31, wherein the conductive structure comprises a conductive pillar structure comprising an under bump metallurgy (UBM) layer, a copper layer, a solder cap and a conductive buffer layer between the copper layer and the solder cap.
 36. The semiconductor package of claim 35, wherein the UBM layer contacts an exposed portion of a metal pad of the semiconductor device, and wherein the solder cap is in contact with at least a portion of the top surface of the base, at least one side wall of the recess and the top surface of the copper trace.
 37. The semiconductor package of claim 31, wherein the conductive structure is in contact with a portion of the top surface of the base and with a side wall of the recess. 